This invention relates to a passive mode lock fiber laser for generating ultra-short light pulses, which is stable against environment temperature change.
Conventionally, for example, a passive mode lock fiber laser is disclosed in Japanese patent laid open publication No. 1996-51246, the passive mode lock fiber laser includes a compensating means for compensating a linear phase fluctuation in a laser cavity and a gain medium, which is difficult received an effect against an environment temperature fluctuation. An embodiment of the passive receiving mode lock fiber laser is disclosed in an optronics 2001. 4 No. 232, pages 153-157 (published on Apr. 10, 2001), which is constructed by Fably-Perot type laser cavity. Concretely, each Faraday rotator which rotates a deflection surface is disposed at a vicinity of both ends of the laser cavity, thereby it is able to obtain a stable liner phase delay between two deflection intrinsic modes when a laser transmits in the optical fiber and interferes with each other therein, and it is able to maintain a stable mode lock condition against the environment temperature change.
However, a repetition frequency is primarily determined by effective length of the laser cavity in the above-mentioned passive mode lock fiber laser. The effective length of the laser cavity fluctuates by the environment temperature change. This is, a portion composing of materials except the optical fiber in the cavity (e.g. base member etc. mounting and fixed with optical parts (e.g. a focus lens, a mirror e.t.c) are composed of metal materials in addition to be occurred an expansion and contraction by the environment temperature change in the optical fiber itself. Generally, compared with glass materials constituting the optical fiber on a linear expansion coefficient, metal materials are larger single digit than glass materials, thereby the expansion and contraction occurs at a portion constituting by the material except the optical fiber. Accordingly, the repetition frequency of the above mentioned fiber laser fluctuates by the environment temperature change.
Actually, in the above-mentioned construction, FIG. 5 shows an embodiment which measured timely transition of the repetition frequency fluctuation under an environment in a generally laboratory when the repetition frequency of the laser is adjusted at, about 50 MHz. In FIG. 5, an abscissa axis indicates hour, and an ordinate axis indicates repetition frequency (KHz) and room temperature (xc2x0C.). According to the measured result shown in FIG. 5, when the room temperature fluctuates about 7xc2x0 C., FIG. 5 shows that the repetition frequency fluctuates by 3 KHz. In variable fields of recent industrial world, under increasing demand of the laser of which is able to maintain a stable mode lock condition against the environment temperature change and it is able to output a constant repetition frequency against the environment temperature change, the laser of which the repetition frequency fluctuates by 3 KHz when the room temperature fluctuates about 7xc2x0 C., in the industrial field of which the laser is applied, as a fluctuation width of the repetition frequency is larger, the laser is not able to use in the industrial field. Otherwise, the laser is not able to use for experiments etc. spent a long time even if the laser is used, and use of the laser is restricted only experiment within a short time under a condition of which the environment temperature change and the fluctuation of the repetition frequency are little.
Further, the above mentioned passive mode lock fiber laser is disclosed in 1570 OPTICS LETTERS/Vol. 21, No. 19/Oct. 1, 1996, which is a constitution of which a position in an optical axis direction of one of mirrors disposed at both end portions of the laser cavity is adjusted by using piezoelectric element (piezo element) so as to actively adjust the cavity length and the repetition frequency. The fiber laser measures a difference between the master laser and the slave laser on the repetition frequency, the fiber laser controls the piezo element in a direction of which the difference of the repetition frequency is decreased by a feedback electronic circuit so as to correspond a repetition frequency of a slave laser adjustable by using the piezo element for the position in the optical axis of one of the mirrors with same frequency based on a fluctuation of a repetition frequency of other laser (e.g. master laser) of which a position control of the mirror is not executed by using the piezo element.
Further, the environment temperature changes of both lasers make same degree by winding an optical fiber constructing the master laser and an optical fiber constructing the slave laser for a same fiber spool, thereby the difference of repetition frequency of the both lasers is suppressed a minimum level. Generally, as a position movable area moving by piezo element is less or equal few hundred micron (xcexc), when the difference of the repetition frequency of the both lasers is greater, it is beyond the movable area of the piezo element. As a result, the repetition frequency of the slave laser is not able to correspond with the repetition frequency of the master laser.
However when an absolutely independent laser pulse signal or electric signals or the like are measured in many application fields using the passive mode lock fiber laser, there is a demand which want to stable against the repetition frequency of the laser under a condition generating the repetition frequency including a fluctuation width larger than the environment temperature change. On this case, it is not able to correspond to only adjustment by the above mentioned piezo element. For example, as the above-mentioned embodiment, when environmental temperature fluctuates by 7xc2x0 C., and when a laser fluctuating the repetition frequency by 3 KHz is disposed under a condition of which environment temperature fluctuates about 50xc2x0 C., the repetition frequency fluctuates about 21 KHz. Hereupon, a calculation formula of which change the repetition frequency to a fluctuation of the cavity width shows hereinafter. When a laser cavity length defines L (m), a light velocity defines c (m/s), an effective refraction factor defines n, a repetition frequency C (Hz) defines C=c/(2 nL). If a refractive index of air defines xe2x80x981xe2x80x99, when the fluctuation width of the cavity width corresponding to the repetition frequency fluctuation by 21 KHz according to the formula is calculated, the fluctuation width thereof results in 1.26 mm. The calculation result means that a necessary fluctuation width of the cavity length needed so as to absorb the repetition frequency fluctuation (21 KHz) is 1.26 mm. When the environment temperature change as this like is large, it is beyond the adjustable area (movable area) by adjusting a position of the mirror using the piezo element which is less than few hundred micron results in an out of adjusting area, thereby it is not stable against the repetition frequency of the laser.
In view of the foregoing, it is an object of the present invention to provide a passive mode lock fiber laser of which mode lock is maintained under a stable condition against environment temperature change, and which is easy to stable adjust against a repetition frequency under a condition of which environment temperature change is, large.
According to a first aspect of the present invention, a passive mode lock fiber laser including an energy generating means for generating a laser energy with a cavity including a gain medium made of an optical fiber for amplifying the laser energy in the cavity, a reflect means for reflecting the laser energy along an optical axis passing through the gain medium, a phase fluctuation compensating means for compensating a linear phase fluctuation of the gain medium, and an output means for outputting the laser energy generated in the cavity includes a temperature adjusting mechanism for adjusting a temperature of an optical fiber portion, and a piezo element position adjusting mechanism for adjusting a position in the optical axis of the reflect means by using a piezo element.
Thereby, the passive mode lock fiber laser includes the piezo element position adjusting mechanism for adjusting the position of the light axis direction of the reflect means by using the piezo element and further includes at least the temperature adjusting mechanism for adjusting the temperature of the optical fiber portion. Accordingly, under the condition of which the environment temperature change is large, the optical fiber portion is difficult to receive effects caused by the environment temperature change. Further, as the fluctuation width of the temperature at the optical fiber portion is at least decreased, the fluctuation width caused by the environment temperature change of the cavity length decreases. Thereby, using the temperature adjusting mechanism, the fluctuation width caused by environment temperature change of the cavity length is able to suppress within an adjusting area of the cavity length (less than few hundred micron) by the piezo element position adjusting mechanism under the condition of which the environment temperature change is large. Then, a fine adjustment of the cavity length is performed by fine adjusting the position in the optical axis direction of the reflect means by the piezo element position adjusting mechanism. According to the present invention, the passive mode lock fiber laser can be surely maintain the cavity length under a certain condition, which is able to stable generate a requested repetition frequency even if the environment temperature change is large. Further, the temperature adjusting mechanism may be adjust a temperature at the optical fiber portion, which further adjusts the temperature at a portion (e.g. base member fixed with optical parts (e.g. focus lens, mirror etc.) consisting of materials except the optical fiber in the cavity.
The above-mentioned temperature adjusting mechanism may be include a heater disposed at vicinity of the optical fiber portion, and include Peltier element disposed at the vicinity of the optical fiber. Thereby, the temperature adjusting mechanism can be established by a simply construction. By the way, when the heater is used, a set temperature of the heater is preferred set more than the room temperature. Further, when the Peltier element is used, the set temperature is preferable set at a close temperature for a room temperature.
Further, according to a second aspect of the present invention, a passive mode lock fiber laser including an energy generating means for generating a laser energy with a cavity including a gain medium made of an optical fiber for amplifying the laser energy in the cavity, a reflect means for reflecting the laser energy along an optical axis passing through the gain medium, a phase fluctuation compensating means for compensating a linear phase fluctuation of the gain medium, and an output means for outputting the laser energy generated in the cavity includes a mechanical position adjusting mechanism for mechanically adjusting a cavity length of the cavity and a piezo element position adjusting mechanism for adjusting a position in the optical axis of the reflect means by using a piezo element.
Hereupon, the mechanical position adjusting mechanism does not include a mechanism using physical characteristics (e.g. expansion and contraction) such as the piezo element position adjusting mechanism, for example, the mechanical position adjusting mechanism means a mechanical mechanism so as to spatially and relatively be able to move mechanical position adjusting mechanism for other components in the present laser.
Thereby, under the condition of which the environment temperature, change is large, even if the fluctuation of the cavity length based on the expansion and contraction caused by the environment temperature change at the optical fiber portion and the portion consisting of materials except the optical fiber portion in the cavity occurs, the cavity length is suppressed within the adjusting area (less or equal few hundred micron) by using of the mechanical position adjusting mechanism, then the fine adjustment of the cavity length can be performed by the fine adjusting the position in the optical axis direction of the reflect means by the piezo element position adjusting mechanism. That is, the temperature adjusting mechanism for rough adjusting the cavity length corresponds to xe2x80x98mechanical position adjusting mechanismxe2x80x99 in the above-mentioned invention. Accordingly, as the cavity length is able to securely maintain certain length under the condition of which the environment temperature change is large in the present invention, it is provided a passive mode lock fiber laser of which the requested repetition frequency is generated under a stable condition.
By the way, the mechanical position adjusting mechanism may be constructed by a movable moving member in the optical direction mounting the reflect means and the piezo element position adjusting mechanism, herewith as a position of the moving member in the optical direction is controlled, it is easy the position thereof to relatively move for another construction element of the laser, it is able to more easy rough adjust the cavity length.